1. Field of the invention
The present invention relates to a process for working a .beta. titanium alloy for improving the isothermal workability of the .beta. titanium alloy by forming fine crystalline grains.
2. Description of the Prior Art
A .beta. titanium alloy called the quasi-stable .beta. titanium alloy, which does not undergo a martensitic transus by chilling and takes a .beta. single phase at room temperature, is generally superior in cold workability, This alloy is cold rolled into thin sheets and then subjected to a solution heat treatment in order to remove a work strain for use in sheet forming.
In general, this alloy has a .beta. transus nearly identical with a recrystallization temperature, and recrystallization and grain growth rapidly occur by heat treatment at the .beta. transus temperature or higher. However, no recrystallization occurs by heat treatment at the .beta. transus temperature or lower while an .alpha. phase is precipitated along a deformation zone and the like formed in working. Accordingly, conventional solution heat treatment of the .beta. titanium alloy has been conducted at a temperature slightly higher than the .beta. transus and particle diameters of crystalline grains obtained by the treatment have been at most about 20 .mu.m [reported in, for example, the Bulletin of Investigations in the Faculty of Engineering of Ibaragi University., 37, 155 (1989)].
The sheet formation is generally conducted by forming in a die at room temperature utilizing the superior cold workability of the .beta. titanium alloy. Similarly, thick sheets have been obtained by die-forging at room temperature. The hot die-forging and the hot sheet forming have been partially conducted at a solution heat treatment temperature or higher in addition to the cold forming.
Although large deformation of the .beta. titanium alloy by the prior art cold working process is possible without generating edge cracking and surface cracking because of excellent cold workability of the alloy, work hardening occurs requiring that an intermediate annealing step be added and thus the number of steps has been disadvantageously increased. In order to solve this problem, hot working has been conducted at a temperature higher than the solution heat treatment temperature to reduce the deformation stress. However, in the hot working and the isothermal working according to the prior art, the .beta. titanium alloy is heated to a temperature higher than the solution heat treatment temperature, so that grains are apt to grow and thus the surface of the formed product tends to be rough. In addition, the formation of coarse particles may have an adverse effect on the mechanical properties after the working. Furthermore, in the conventional hot working and the isothermal working, the material was not largely deformed at a reduced stress and near final shaping was extremely difficult.
In order to solve the above-described problems, according to the present invention, a .beta. titanium alloy is subjected to elongating, aging and elongating in the order described to form fine crystalline grains during the time when it is heated to the isothermal working temperature and held at that temperature and thus a super-plasticity capable of undergoing large deformation with a reduced stress can be exhibited. Such a super-plastic phenomenon can be utilized for not only a reduction in the manufacturing costs but also the diversification in design due to superior transferability and diffusion joinability.